1. Field of the Invention
The present disclosure relates to an image forming apparatus, such as an electrophotographic copying machine and an electrophotographic printer.
An electrophotographic image forming apparatus forms an image on a recording medium (recording sheet) by using the electrophotographic image forming process. Electrophotographic image forming apparatuses include, for example, electrophotographic copying machines, electrophotographic printers (such as laser beam printers and light emitting diode (LED) printers), facsimile machines, and word processors.
2. Description of the Related Art
With the progress of the information society in recent years, the needs for color image forming apparatuses have been increasing, and increasing number of full color image forming apparatuses (such as color copying machines and color printers) for outputting color images have been put into practical use.
Such a full color image forming apparatus includes four image forming stations corresponding to four colors (yellow, magenta, cyan, and black) disposed in a row in this order. Toner images formed on photosensitive drums (image carriers) of respective image forming stations are primarily transferred onto an intermediate transfer member in sequence so that the toner images are placed on top of each other. Thus, a 4-color toner image is formed on the intermediate transfer member. Then, the 4-color toner image is secondarily transferred onto a recording medium to acquire an output image. This process is referred to as in-line process. Full color image forming apparatuses employing the in-line process are widely used.
In each image forming station, a charging device, such as a charging roller in contact with a photosensitive drum, charges the photosensitive drum. Then, an image exposure device forms an electrostatic latent image according to the image information on the photosensitive drum surface. A developing device storing toner (developer) develops the electrostatic latent image into a visible toner image of each color.
Visible toner images formed on the photosensitive drum surfaces in respective image forming stations are primarily transferred onto the intermediate transfer member in sequence so that the toner images are placed on top of each other. Thus, an unfixed 4-color (yellow, magenta, cyan, and black) full color toner image is formed on the intermediate transfer member.
The full color toner image is secondarily transferred from the intermediate transfer member onto a recording medium. Then, a fixing device heats and pressurizes the toner image to fix it to form a recorded image. After the primary transfer onto the intermediate transfer member, a cleaning unit having a cleaning blade collects primary transfer residual toner remaining on the photosensitive drum surface as waste toner. Thus, the photosensitive drum surface is cleaned, and prepares for the next image formation.
As the developing device, the contact developing method is widely used, in which a developing roller made of elastic rubber is brought into contact with the photosensitive drum to develop the electrostatic latent image on the photosensitive drum.
Each image forming station may be configured as a process cartridge which integrates any one or all of the photosensitive drum, the charging device, the developing device, and the cleaning unit, and is easily detachably attached to the image forming apparatus.
A full color image forming apparatus is provided with the full color mode in which image formation is performed by using toners of a plurality of colors to output a full color image, and the mono color mode in which image formation is performed by using only monochromatic (black) toner to output a mono color image. In the mono color mode, ideally, the photosensitive drums and developing devices for non-black colors are deactivated to avoid abrasion of these members.
In this case, however, relevant drive units will be complicated possibly resulting in an increase in size and cost of the apparatus. Therefore, in the mono color mode, the developing devices for non-black colors are separated from respective photosensitive drums and deactivated, but the photosensitive drums for all colors may be operating with the charging bias voltage applied thereto.
Further, with a process cartridge type image forming apparatus, when a consumable, such as a photosensitive drum and toner, reaches or comes close to the end of the life, it is necessary to inform a user of the relevant fact to allow the user to replace the relevant cartridge with a new one at an appropriate timing.
As a photosensitive drum, an organic photosensitive member composed of a supporting member, and a photosensitive layer (organic photosensitive layer) formed thereon is widely used because of advantages of low price and high productivity. The photosensitive layer uses organic materials as photoconductive materials (charge generating material and charge transport material). As an organic photosensitive member, a photosensitive drum formed of laminated photosensitive layers is mainly used because of advantages of high sensitivity and diversity in material design. The laminated photosensitive layers include a charge generation layer containing a charge generating material and a charge transport layer containing a charge transport material.
In many cases, various types of layers are provided between the supporting member and the photosensitive layer to coat the surface of the supporting member, improve the coating properties of the photosensitive layer, improve the adhesiveness between the supporting member and the photosensitive layer, protect the photosensitive layer from electrical damages, improve the charging properties, improve the properties of charge injection from the supporting member to the photosensitive layer, and so on.
Providing between the photosensitive layer and the supporting member a conductive layer for coating the surface of the conductive supporting member, and an intermediate layer having electrical barrier properties for preventing charge injection from the conductive layer to the photosensitive layer enables acquiring a photosensitive drum having stability in manufacturing and quality. As bonding resin for the charge transport layer of the photosensitive drum, polycarbonate resin and polyarylate resin for improving mechanical strength are widely used.
A common photosensitive drum is formed of a resistive layer, an under coat layer, a charge generation layer, and a charge transport layer sequentially laminated on the conductive supporting member by using the dipping coating method. In the above-described image formation process, the photosensitive drum is subjected to electrical and mechanical external forces, such as discharging process due to charging, sliding friction by the developing device and the intermediate transfer member, and scratching by the cleaning blade. As a result, the charge transport layer (hereinafter referred to as CT layer) abrades away and wears down with operating time of the image forming apparatus. Therefore, in many cases, the life of the photosensitive drum is determined by the amount of remaining film thickness of the CT layer (hereinafter referred to as remaining CT film thickness).
Accordingly, there have been proposed various techniques for predicting the amount of CT layer abrasion with operating time of the photosensitive drum, and determining the life of the photosensitive drum within a range in which the levels of abrasion unevenness and fogging do not decrease. Japanese Patent Application Laid-Open No. 2001-356655 discusses a technique for comparing an integrated value integrating the time of voltage application to a photosensitive drum by a charging device and the time of contact of a developing device with the photosensitive drum with predetermined life information (photosensitive drum life threshold value) to predictively determine the life of the photosensitive drum. The technique discussed in Japanese Patent Application Laid-Open No. 2001-356655 predictively determines the life of the photosensitive drum based on the remaining CT film thickness in an image forming region on the photosensitive drum.
However, with an image forming apparatus having a plurality of image formation execution modes, such as the above-described full color and mono color modes, the abrasion state of the photosensitive drum differs for each color mode. Thus, the following problem arises.
With demands for decreasing the size and cost, and simplifying the configuration of an image forming apparatus in recent years, a process cartridge attached to the image forming apparatus is demanded to decrease in size. To miniaturize the process cartridge, members included therein also need to be smaller. To reduce the size, it is also important to reduce the length of each member in the axial direction (longitudinal direction). When adopting such a configuration, both longitudinal ends of the developing roller and the charging roller in contact with the photosensitive drum are disposed at close positions on the photosensitive drum surface.
In such a configuration, the amount of abrasion of the photosensitive drum is not uniform in the longitudinal direction of the photosensitive drum. More specifically, at both longitudinal ends of the photosensitive drum, the CT layer abrasion is promoted in regions where the ends of the developing roller and the ends of the charging roller are disposed at close positions.
Specifically, a toner non-application region exists at both longitudinal ends of the developing roller. Therefore, the abrasion resulting from mechanical stress onto the photosensitive drum by the ends in the toner non-application regions of the developing roller overlap the abrasion resulting from increased amount of discharge at the end faces of the charging roller. Therefore, with the photosensitive drum, the amount of CT layer abrasion at both longitudinal ends is larger than the amount of CT layer abrasion in the image forming region used for image formation.
An increase in the amount of abrasion at both longitudinal ends of the photosensitive drum may abrade all of the charge transport (CT) layer, the charge generation layer, and the under coat layer, and the abrasion may reach the resistive layer. In this case, since the charging bias voltage applied to the developing roller and the charging roller leaks to the resistive layer, fogging may be produced due to charging failure or image may be missed due to development failure. In the above-described image formation process, this phenomenon appears more notably in the full color mode in which the developing roller is constantly in contact with the photosensitive drum.
Therefore, in the full color mode, to prevent leak at both longitudinal ends of the photosensitive drum, the time immediately before the CT layer abrasion at both longitudinal ends of the photosensitive drum reaches the resistive layer is set as the life of the photosensitive drum. The life of the photosensitive drum has been predicted by presetting as a photosensitive drum life threshold value the CT layer film thickness in the image forming region of the photosensitive drum at this timing, and performing control like discussed in Japanese Patent Application Laid-Open No. 2001-356655.
However, in this case, the CT layer film thickness in the image forming region set as the photosensitive drum life threshold value may be sufficient for performing image formation. Specifically, there has been a case where, in the image forming region of the photosensitive drum, a CT layer film thickness larger than the CT film thickness at which an image failure occurs may be set as the life of the photosensitive drum.
With the developing devices for non-black colors in the mono color mode, the developing roller is not in contact with the photosensitive drum, as described above. Therefore, at both longitudinal ends of the photosensitive drum, only the abrasion at the end faces of the charging roller affects the life. Then, with the photosensitive drums for non-black colors in the mono color mode, the CT layer abrasion, which is promoted when the ends of the developing roller and the ends of the charging roller are close to each other, does not occur at the above-described longitudinal ends.
Therefore, when printing is performed only in the mono color mode, with the photosensitive drums for non-black colors, the CT layer film thickness immediately before an image failure occurs in the image forming region can be set as the life of the photosensitive drum without being affected by the CT layer abrasion at both longitudinal ends of the photosensitive drums. In this case, of course, image formation can no longer be continued.
Therefore, with the photosensitive drums for non-black colors of the full color image forming apparatus, when image formation is performed only in the full color mode, it was necessary to set a large value of the remaining CT film thickness in the image forming region to be set as the photosensitive drum life threshold value. When image formation is performed only in the mono color mode, it is necessary to set a small value of the remaining CT film thickness in the image forming region to be set as the photosensitive drum life threshold value.
However, with the above-described conventional image forming apparatus, the remaining film thickness in the image forming region in a certain specific mode is set as the photosensitive drum life threshold value, although the image forming apparatus is provided with a plurality of modes. In all modes, the image forming apparatus predicts the life of the photosensitive drums by using the one photosensitive drum life threshold value. For this reason, there have been a case where the photosensitive drum is determined to have reached the end of the life although image formation is still possible, and a case where the photosensitive drum is continuously used although image formation is no longer possible.